Since my Prius is parked on the street outside in the sun I would find my Prius 12 volt battery would be too dead to start my car. Here is the parts list. Look at this on eBay 100A Solar Panel Charge Controller 12V 24V Regulator Auto Mppt Battery Dual USB | eBay Look at this on eBay 30W 18V Outdoor USB Monocrystalline Solar Panel Battery Charger Motorhome | eBay These parts added up to about $40 and now is part of my solar power trickle charger keeping my 12 volt power full and healthy. The PWM controller can handle up to 200 watt solar panel and keep my ARB fridge running all day when I'm out for a long weekend camping. It was easy to install and has many features to maintain battery of all types lithium, Lead Acid and AGM. It also has two usb ports for charging devices. I will upload photos of my set up and show you various ways for the small panel location.
Be careful and keep an eye on the battery if you leave it parked for more than 3 days. I have seen reviews that these panels can not keep up and the battery ends up draining anyway. I tapped into my factory solar panel to do the same thing and keep the battery charged. http://www.RedBullet.net http://www.ProjectLithium.com http://www.Pulstar.com http://www.PlugOutPower.com
Can it keep up? Good question. How big is that panel, maybe 12 cm x 30 cm? Solar power (total from the Sun at the Earth) is around 1KW per square meter, and solar panel efficiency is around 20% (.2). So the power from a panel like that, is at most 1000 * .12 * .3 * .2 = 7W. Assuming the electronics are 100% efficient, the corresponding current is 7W/12V = 583 mA. So, under ideal circumstances, that is more than enough to offset the 15-20 mA car off current, even though that off current is 24 h/day whereas the charging current is more like 10 h/day. The thing is, that panel is not likely to be in ideal conditions unless the owner moves it around during the day. The rule of thumb is, I believe, that panels which have indirect sunlight put out about half as much power as those in direct sunlight. If the panel is not built well a dark shadow (neither direct nor indirect light) falling on just one part could drop the output to next to nothing. This is because no current flows in these devices unless they are in light. The poorly made ones just put a bunch of solar cells in series, and if any one cell sees no light it shuts off current flow through the whole chain. Better made ones use diodes to shunt current from still working cells around the ones which are off. Those just lose power more or less as a function of the area which is shaded, it isn't all or nothing. There are also geometry issues - the calculation above only applies if the light is parallel to the panel's normal vector. There is likely a loss factor of somewhere between .5 and .7 because of this. The glass will also block some of the light, so maybe another factor of .5? Rough calculations, but it suggests that a small panel like that, if it is of high quality, could keep the 12V charged as long as the car was parked in a reasonably sunny location.
